1. Field of the Invention
The present invention relates to a method and apparatus for constructing a synchronous sensor network, and more particularly, to a method and apparatus for constructing and using a sensor network by using a power beacon in order to construct a ubiquitous sensor network (USN) system by using a synchronous sensor network medium access control (MAC) protocol such as a ZigBee or IEEE 802.15.4 low-rate wireless personal area network (WPAN).
2. Description of the Related Art
A ubiquitous sensor network (USN) constructs a wireless sensor network by using a plurality of sensor nodes each having a sensor for sensing object recognition information or environmental information.
The USN refers to a network system that processes and manages information input from various sensors of a wireless sensor network in real-time, in association with an external network.
The USN aims to ultimately realize an environment that can communicate anytime and anywhere regardless of a network type, a device type, or a service type by providing computing and communication functions to every object.
FIGS. 1A through 1C are diagrams showing a conventional synchronous USN such as IEEE 802.15.4 or ZigBee.
Referring to FIGS. 1A and 1B, a personal area network (PAN) coordinator 101 is a node representing the USN, manages wireless resources of the USN, and associates the USN with an external network.
Coordinators 102, 103, 104, and 105 collect information by using sensors and route sensor data collected by device nodes or end nodes 106, 107, 108, and 109.
In FIG. 1A, dashed circles 101-1, 102-1, 103-1, 104-1, and 105-1 respectively refer to sensor fields representing physical propagation ranges of signals output from the PAN coordinator 101 and the coordinators 102, 103, 104, and 105.
The conventional synchronous USN forms logical parent-child associations between nodes.
Referring to FIGS. 1A and 1B, the PAN coordinator 101 is a parent of the coordinator 102 that is a parent of the coordinator 104 that is a parent of end nodes 108. Information collected by the end nodes 108 is transmitted to the PAN coordinator 101 through the coordinator 104 and the coordinator 102.
The PAN coordinator 101 ultimately transmits the collected information to the external network.
Parent-child association is formed as logical association and sensor data is transmitted through associated nodes.
Although the propagation range of a signal output from the coordinators 108 covers the coordinator 102, the signal is transmitted to the coordinator 102 through the coordinator 104.
In the conventional synchronous USN, one coordinator forms one sensor field together with a plurality of device nodes.
FIG. 1C shows a medium access control (MAC) protocol for maintaining minimum power consumption in the conventional synchronous USN.
Referring to FIG. 1C, in order to efficiently consume power, each sensor node communicates by minimizing power consumption in inactive periods that are not used, and supplying power only in active periods.
The active and inactive periods of each sensor node are formed by transmitting a beacon packet set by the PAN coordinator 101.
The beacon packet set by the PAN coordinator 101 is transmitted to child nodes of the PAN coordinator 101, and the child nodes operate in active and inactive periods in time-synchronization with wireless active and inactive periods included in the beacon packet.
As illustrated in FIGS. 1A and 1B, periods for enabling communication (active periods) and periods for disabling communication (inactive periods) are set by transmitting the beacon packet to the coordinators 102 and 103 of the PAN coordinator 101.
As such, the active and inactive periods are set in each of the sensor fields formed between the coordinators 102, 104, and 106, between the coordinators 104 and 108, between the coordinators 103, 105, and 107, and between the coordinators 105 and 109.
In the structure illustrated in FIGS. 1A through 1C, due to the beacon packet routed from the PAN coordinator 101 to the coordinator 102 and to the coordinator 104, and from the PAN coordinator 101 to the coordinator 103 and to the coordinator 105, a serious delay occurs to an initial state of the conventional synchronous USN.
If the number of nodes each forming a sensor field is increased and thus the number of hops is increased, the delay of the beacon packet from a synchronization node to an end node is greatly increased.
The delay of the beacon packet causes inaccurate time synchronization in periods for enabling data transmission (active periods) and periods for disabling data transmission (inactive periods), so that a reliable network may not be obtained.
In the structure illustrated in FIGS. 1A through 1C, if emergency data is generated in the end node 108, the emergency data is synchronized with an active period and is ultimately transmitted to the PAN coordinator 101 through routing nodes such as the coordinator 104 and the coordinator 102, and thus a great delay occurs.